After unmasking himself publicly to prove a point, Peter Parker/Spider-Man is heartbroken when a hitman hired by a super-villan later tries to kill him only to miss and instead hit his Aunt May.

He then spent a time desperately running around the Marvel Universe trying to find someone who can heal a simple bullet-wound with no luck, despite there being countless supers who can bloody rewrite reality itself with just their minds! In his last ditch effort he makes a deal with a powerful daemon who bargins the life of Aunt May for the marriage of Peter Parker and Mary Jane....

For some reason seeing them as a couple makes him sick, so the deal is that he will save Aunt May at the price of rewriting all of reality so that Peter Parker never married Mary Jane. So like a fool Peter Parker signs this devil-deal and the entirety of the Marvel Universe is rewritten so that they never married....

you should have added that the only thing that changed is literally they never married. They still sleep together and are a couple and everything so, like TV tropes says, all that "Totally-not-Satan" guy took was some wedding photos and a piece of paper.